Photovoltaic Power Plant Output

ABSTRACT

A photovoltaic power system can include a photovoltaic array, an inverter, and a battery.

CLAIM FOR PRIORITY

This application claims priority under 35 U.S.C. §119(e) to ProvisionalU.S. Patent Application Ser. No. 61/152,345 filed on Feb. 13, 2009, andis a continuation of Application No. PCT/US10/24240, filed Feb. 15,2010, each of which is incorporated by reference in its entirety.

TECHNICAL FIELD

This invention relates to photovoltaic power plant output.

BACKGROUND

Photovoltaic power plants are becoming practical as grid scalegeneration facilities capable of producing tens of megawatts, decreasingthe cost of photovoltaic modules. Larger plants are being built tosatisfy mandates for renewable energy capacity. Penetration levels ofphotovoltaic plants are expected to be significant. Photovoltaic plantsmay produce short term, rapidly changing (e.g. as much as 100% of ratedpower per minute or 200% of rated power per minute) variable outputpower that may be due to shading caused by isolated moving clouds. Forthis reason, energy producers and grid operators may consider largephotovoltaic power plants (i.e. larger than 2 MW) to be unpredictable.

Stability, reliability, and power quality of the electric grid, whichmay be negatively affected by large photovoltaic power plantvariability, is controlled by allocating reserve generation capacity forload following, spinning reserve, voltage support, and frequencyregulation with hydro, steam, and combustion turbine generators that canrespond relatively faster than base load power generation plants such ascoal and nuclear type plants. Reserve generation resources may not havethe response characteristics needed to account for short termvariability of large solar photovoltaics.

Reducing short term variability of large solar photovoltaics wouldreduce the need for reserve generation. Large photovoltaic plantsconnected at utility grid distribution level (69 kV and less), mayintroduce voltage deviations and flicker depending upon the capacity andimpedance of the network or feeder. Reducing short term variabilitywould reduce voltage effects and/or permit voltage regulation devices toreact.

DESCRIPTION OF DRAWINGS

FIG. 1 is a graph of global irradiance.

FIG. 2 is a schematic of a photovoltaic power plant with a DC connectedbattery.

FIG. 3 is a schematic of a photovoltaic power plant with an AC connectedbattery.

FIG. 4 is a schematic of a photovoltaic power plant with a generator.

FIG. 5 is a schematic of a photovoltaic power plant with a fuel cell.

FIG. 6 is an illustration graph of photovoltaic plant power output withlow rate of change control.

FIG. 7 is an illustration graph of photovoltaic plant power output withhigh rate of change control.

DETAILED DESCRIPTION

Solar energy plants under favorable weather and cloud conditions tend tobe predictable in that energy production coincides with daily solarcycles, fairly close to daily peak demand profile. Photovoltaic solarenergy, however, is subject to short term variability when isolatedcumulus clouds pass over the photovoltaic array, reducing or eliminatingthe direct incident component of solar radiation. Clouds have a widelyvariable effect, from a 10 or 15% reduction from thin cirrus clouds, toa 50-70% reduction from dense cumulus clouds (thunderheads). Duringcloud passage, the photovoltaic array may produce power only from thediffuse component of solar radiation. FIG. 1 showing global irradianceillustrates the relatively fast changes in solar radiation that arepossible. A photovoltaic plant power output profile is very similar tothe global irradiance profile shown in FIG. 1.

Short-term variability of sufficiently large, multi-megawattphotovoltaic power plants can have negative effects on transmission gridstability, requiring large amounts of regulation and spinning reserveresources to be allocated to account for sudden and large changes inphotovoltaic plant output. Additionally, photovoltaic plants connectedto distribution networks can cause load flow, voltage flicker, andvoltage regulation problems for loads on the network and may causeconditions that could trip network protective devices.

Short term variability can be reduced by including an auxiliary powersource as part of the photovoltaic plant. The auxiliary power source maybe a stored energy system such as an electrolytic cell type device suchas a capacitor battery (e.g. lead-acid, nickel-cadmium, sodium, orlithium-ion battery), or flow battery, as is known in the art. Theauxiliary power source may also be an alternative generation device suchas a fuel cell or generator driven by flywheel and/or prime mover fueledby a gas or liquid and/or compressed air. The auxiliary power source maybe a system comprised of two or more of the types previously described.The auxiliary power source can have the capability of responding topower control signals within seconds and be capable of changing powerlevel at a rate of greater than 100% of photovoltaic power plant ratingper minute or greater than 200% of photovoltaic power plant rating perminute.

This arrangement can control the rate of change of photovoltaic plantoutput power to emulate the more slowly responding characteristics ofthermal power plants, e.g., steam or combustion turbine generation. Aless variable photovoltaic plant output assures efficient and stableresponse to short term solar variability by all available gridgeneration resources. The configuration of the photovoltaic plant forcontrolling photovoltaic power plant output rate of change, alsoreferred to as ramp rate, is illustrated as shown in the Figures anddescribed below.

In general, a method for generating less variable output power from alarge, grid connected photovoltaic plant can include receivingphotovoltaic power from a photovoltaic array, measuring the rate ofchange of photovoltaic power, and adjusting power from a controllableauxiliary power source and the photovoltaic power converter, where theoutput power of combined photovoltaic power and auxiliary power sourceis set to operate within a given power output rate change band whichdefines the maximum allowable positive and negative limits for the plantoutput power rate of change.

Adjusting auxiliary power can include adjusting an auxiliary powersource having the power and energy capacity and dynamic responseappropriate for the system configuration and modes of operation asdescribed herein. Since the potential power output rate of change of aphotovoltaic plant can be relatively rapid, the auxiliary source canhave the capability to respond faster. The auxiliary power can be acontinuously available utility supplied energy source. The auxiliarypower can be a high rate rechargeable source with significantly limitedenergy storage. The auxiliary power can be a low rate rechargeableauxiliary power source with significantly large energy storage. Energystorage systems may include mechanical and/or electrical devices forconverting and storing energy. The stored energy system can be, but isnot limited to, one of an electrolytic cell device, an electrochemicalcell device, or a mechanical kinetic and/or potential energy storagedevice. The stored energy system can include, but is not limited to, anelectrolytic cell device, a capacitor, a lead-acid battery, anickel-cadmium battery, a sodium battery, a lithium-ion battery, a flowbattery, or a mechanical kinetic and/or potential energy storage device.The auxiliary power can be an alternative generation device. Thealternative generation device can be, but is not limited to, one of afuel cell, a wind turbine generator, a flywheel generator, a gas fueledcombustion prime mover-generator, a liquid fueled prime mover-generator,a compressed-gas powered prime mover-generator, or combinations thereof,including an air-powered prime mover-generator. Adjusting the auxiliarypower can include adjusting a stored energy system or an alternativegeneration device or adjusting the amount of stored energy in the storedenergy system to maintain the plant output power rate of change towithin the positive and negative power rate change limits. Adjusting theauxiliary power can include adjusting the optimum combination ofmultiple energy storage device types, multiple power generation devicetypes, or multiple energy storage and power generation devices.

Adjusting the auxiliary power can include adjusting a source that iscontinuously available. The auxiliary power can be a utility suppliedenergy source. The auxiliary power can be a source with significantlylimited energy source or a high rate rechargeable source. Adjusting theauxiliary power can include adjusting a stored energy system or analternative generation device. The stored energy system can be, but isnot limited to, one of an electrolytic cell device, a capacitor, alead-acid battery, a nickel-cadmium battery, a sodium battery, alithium-ion battery, a flow battery, or a mechanical kinetic and/orpotential energy storage device. Energy storage and alternativegeneration systems may include mechanical and/or electrical devices forconverting and storing energy. The alternative generation device can be,but is not limited to, one of a fuel cell, a wind turbine generator, aflywheel generator, a gas fueled prime mover-generator, a liquid fueledprime mover-generator, or a compressed-air powered primemover-generator.

If the auxiliary power source is a stored energy system, the method caninclude increasing an amount of stored energy in the auxiliary powersource when the photovoltaic output power rate of change exceeds thepositive limit of the power output rate change band. The method caninclude decreasing photovoltaic power when the photovoltaic output powerrate of change exceeds the positive limit of the power output ratechange band. The method can include maintaining a constant amount ofstored energy in the auxiliary power source when the photovoltaic outputpower rate of change is within the allowable power output rate changeband. The method can include decreasing an amount of stored energy inthe auxiliary power source when the photovoltaic output power rate ofchange exceeds the negative limit of the power output rate change band.The method can include establishing secondary positive and negativepower rate change limits which have a tighter tolerance than the normalpower rate change limits. The method can include establishing one ormore set points defining the upper and lower capacity limits of thestored energy in the auxiliary power source used to determine thetransition from the normal to the secondary power rate change limits.The method can include setting upper and lower capacity hysteresislimits of the stored energy in the auxiliary power source used todetermine the transition from the secondary to the normal power ratechange limits. The method can include switching to the secondarypositive power rate change limit when the level of stored energy fallsbelow the lower capacity limit to facilitate faster charging of theauxiliary power source when the output power rate of change is positive.The method can include returning to the normal positive power ratechange limit when the level of stored energy in the auxiliary powersource rises above the lower capacity hysteresis limit. The method caninclude switching to the secondary negative power rate change limit whenthe level of stored energy rises above the upper capacity limit tofacilitate faster discharging of the auxiliary power source when theoutput power rate of change is negative. The method can includereturning to the normal negative power rate change limit when the levelof stored energy in the auxiliary power source falls below the uppercapacity hysteresis limit.

The method can include operation of the plant to dispatch plant outputpower following a dispatch set point. The method can includeestablishing a dispatch set point that is constant. The method caninclude establishing a dispatch set point derived from a scheduleconsisting of a series of different points. The method can includeincreasing the level of stored energy in the auxiliary power source whenthe photovoltaic output power is greater than the dispatch set point.The method can include decreasing the level of stored energy in theauxiliary power source when the photovoltaic output power is less thanthe dispatch set point. The method can include limiting the rate ofchange of the combined photovoltaic power and auxiliary power source tooperate within the power output rate change band when the dispatch setpoint is changed.

If the auxiliary power source is an alternative power device, the methodcan include maintaining constant auxiliary power when the photovoltaicoutput power rate of change is within the allowable power output ratechange band. The method can include reducing auxiliary power when thephotovoltaic output power rate of change exceeds the positive limit ofthe power output rate change band. The method can include increasingauxiliary power when the photovoltaic output power rate of changeexceeds the negative limit of the power output rate change band. Themethod can include decreasing photovoltaic power when the photovoltaicoutput power rate of change exceeds the positive limit of the poweroutput rate change band. The method can include maintaining plant outputpower at a constant level if photovoltaic output power increases ordecreases.

The method can further include inertial set points to suppress the rateof plant output power rate of change when transitioning to a differentpower rate change condition including a negative power rate change to apositive power rate change condition or transitioning from a positivepower rate change to negative power rate change condition. The methodcan further include using the plant irradiance measurement as ananticipatory indicator in determining when a change in the power outputrate change should be made and the approximate new value of the powerrate change limits. The method can further include slowing or reducingthe plant output rate of change when the plant measured irradiance valuereaches a relatively stable minimum or maximum value. The method canfurther include adjusting the rate at which stored energy is increasedin the stored energy system, up to the allowable positive power ratechange limit, if the irradiance measurement is increasing. The methodcan further include adjusting the rate at which stored energy isdecreased in the stored energy system, up to the allowable negativepower rate change limit, if the irradiance measurement is decreasing.The method can further include adjusting the rate at which auxiliarypower is decreased, up to the allowable positive power rate changelimit, if the irradiance measurement is increasing. The method canfurther include adjusting the rate at which auxiliary power isincreased, up to the allowable negative power rate change limit, if theirradiance measurement is decreasing.

The method can further include establishing plant operation to follow anenergy/power schedule based on a solar energy production forecast oroperation of the plant to follow a predetermined energy/power scheduledetermined by solar energy production forecasting techniques. Theenergy/power forecast schedule may predict the energy and average powerexpected from the plant for each hour of production. The method canfurther include an upper power forecast limit and a lower power forecastlimit based on the average forecasted power within an hour period. Themethod can further include decreasing or increasing the auxiliary poweror the level of stored energy in the stored energy system to maintainthe plant output rate of change within the plant output rate change bandif the plant output power is within the upper and lower power forecastlimits. The method can further include decreasing the level of storedenergy in the stored energy system to maintain constant plant outputpower if the plant output power approaches the lower power forecastlimit. The method can further include increasing the level of storedenergy in the stored energy system to maintain constant plant outputpower if the plant output power approaches the upper power forecastlimit. The method can further include increasing the auxiliary power tomaintain constant plant output power if the plant output powerapproaches the lower power forecast limit. The method can furtherinclude decreasing the auxiliary power to maintain constant plant outputpower if the plant output power approaches the upper power forecastlimit. The method can further include limiting the rate of change of thecombined photovoltaic and auxiliary power source to operate within thepower output rate change band when transitioning from one scheduled hourto the next scheduled hour. The method can further include adjustingauxiliary power or the level of stored energy in the stored energysystem to control the plant output to minimize any revenue penalties dueto the net energy deviation from scheduled based on predeterminedcriteria associated with utility rate structures and tariffs.

The method can include adjusting auxiliary power or the level of storedenergy to compensate for changes in cloud cover or anticipated changesin cloud cover. The method can include increasing auxiliary power or thelevel of stored energy when cloud cover is forecast or physicallydetected. The method can include adjusting auxiliary power or the levelof stored energy in the absence of cloud cover. The method can includereducing auxiliary power or the level of stored energy when no cloudcover is forecast or physically detected. The method can includeadjusting auxiliary power or the level of stored energy for gridfrequency regulation, ancillary services, or load shifting whileproducing less variable output from a large, grid connected photovoltaicplant.

A system for generating less variable output power can include receivinga photovoltaic array, an inverter connected to the photovoltaic array;and an auxiliary power source, connected by a direct current converterto the inverter, where the inverter produces alternating current outputpower to a grid. The system can include a plant control systemcontrolling the inverter and auxiliary power source, measuring the rateof change of power from the photovoltaic array, and adjusting auxiliarypower output. The system can include the output power of the combinedphotovoltaic array and auxiliary power source set to operate within agiven power output rate change band which defines the maximum allowableplant output power rate of change, positive of negative. The system caninclude separate and independently adjustable set points for positiveand negative power rate change limits. The system can include power ratechange limits that are continuously adjustable between 0% (zero) and100%, positive and negative. The system can include set points that canbe pre-set and then automatically changed by the plant control system inresponse to time of day; current, scheduled, or anticipated photovoltaicplant operating conditions; or current or anticipated weatherconditions.

The system can include the auxiliary power source being a source that iscontinuously available. The auxiliary power source can be a utilitysupplied energy source. The auxiliary power source can be a source withsignificantly limited energy source or a high rate rechargeable source.The system can include the auxiliary power source being at least one ofa stored energy system and an alternative generation device. The storedenergy system can be one of an electrolytic cell device, a capacitor, alead-acid battery, a nickel-cadmium battery, a sodium battery, alithium-ion battery, a flow battery, or a mechanical kinetic and/orpotential energy storage device. The alternative generation device canbe one of a fuel cell, a wind turbine generator, a flywheel generator, agas fueled prime mover, a liquid fueled prime mover, or a compressed-airpowered prime mover. The system can include the auxiliary power being asource having the power and energy capacity and dynamic responseappropriate for the system configuration and modes of operation asdescribed herein. Since the potential power output rate of change of aphotovoltaic plant can be relatively rapid, the auxiliary source canhave the capability to respond faster. The auxiliary power can be acontinuously available utility supplied energy source. The auxiliarypower can be a high rate rechargeable source with significantly limitedenergy storage. The auxiliary power can be a low rate rechargeableauxiliary power source with significantly large energy storage. Energystorage systems may include mechanical and/or electrical devices forconverting and storing energy. The stored energy system can be, but isnot limited to, one of an electrolytic cell device, a capacitor, alead-acid battery, a nickel-cadmium battery, a sodium battery, alithium-ion battery, a flow battery, or a mechanical kinetic and/orpotential energy storage device. The auxiliary power can be analternative generation device. The alternative generation device can be,but is not limited to, one of a fuel cell, a wind turbine generator, aflywheel generator, a gas fueled combustion prime mover-generator, aliquid fueled prime mover-generator, a compressed-gas powered primemover-generator, or combinations thereof, including an air-powered primemover-generator.

The system can include the auxiliary power being at least one of astored energy system and an alternative generation device. This systemcan include the optimum combination of multiple energy storage devicetypes, multiple power generation device types, or multiple energystorage and power generation devices. The system can include anauxiliary power source comprising of an energy storage system. The plantcontrol system can increase the level of stored energy in the auxiliarypower source when the photovoltaic output power rate of change exceedsthe positive limit of the power output rate change band. The system candecrease photovoltaic power when the photovoltaic output power rate ofchange exceeds the positive limit of the power output rate change band.The plant control system can maintain a constant level of stored energyin the auxiliary power source when the photovoltaic output power rate ofchange is within the positive and negative limits of the power outputrate change band. The plant control system can decrease the level ofstored energy in the auxiliary power source when the photovoltaic outputpower rate of change is below the negative limit of the output powerrate change band. The system can include secondary positive and negativepower rate change limits which have a tighter tolerance than the normalpower rate change limits. The system can include set points defining theupper and lower capacity limits of the stored energy in the auxiliarypower source used to determine the transition from the normal to thesecondary power rate change limits. The system can include upper andlower capacity hysteresis limits of the stored energy in the auxiliarypower source used to determine the transition from the secondary to thenormal power rate change limits. The plant control system can switch tothe secondary positive power rate change limit when the level of storedenergy falls below the lower capacity limit to facilitate fastercharging of the auxiliary power source when the output power rate ofchange is positive. The plant control system can return to the normalpositive power rate change limit when the level of stored energy in theauxiliary power source rises above the lower capacity hysteresis limit.The plant control system can switch to the secondary negative power ratechange limit when the level of stored energy rises above the uppercapacity limit to facilitate faster discharging of the auxiliary powersource when the output power rate of change is negative. The plantcontrol system can return to the normal negative power rate change limitwhen the level of stored energy in the auxiliary power source fallsbelow the upper capacity hysteresis limit.

The system can include a plant control system controlling the outputpower of the combined photovoltaic array and auxiliary power source tooperate at a constant power level. The system can include separate andindependently adjustable set points for positive and negative power ratechange limits. The system can include set points limiting the plantpower output rate of change that are continuously adjustable between 0%(zero) and 100%, positive and negative. The plant control system canoperate the plant to maintain a constant dispatch set point. The systemcan operate the plant to maintain a dispatch set point that is constant.The system can operate the plant to follow a schedule consisting of aseries of different dispatch set points. The system can increase thelevel of stored energy in the auxiliary power source when thephotovoltaic plant output is greater than the dispatch set point. Thesystem can lower the level of stored energy in the auxiliary powersource when the photovoltaic plant output is less than the dispatch setpoint. The system can limit the rate of change of the combinedphotovoltaic power and auxiliary power source to operate within thepower output rate change band when the dispatch set point is changed.

The system can include an auxiliary power source comprising of a powergeneration device. The plant control system can reduce the power outputfrom the auxiliary power source when the photovoltaic power rate ofchange exceeds the positive limit of the power output rate change band.The plant control system can maintain a constant power output from theauxiliary power source when the photovoltaic power rate of change iswithin the positive and negative limits of the power output rate changeband. The plant control system can increase the power output from theauxiliary power source when the photovoltaic power rate of change isbelow the negative limit of the power output rate change band. The plantcontrol system can decrease photovoltaic power when a photovoltaicoutput power rate of change exceeds a positive limit of the power outputrate change band. The plant control system can maintain output power ata constant level if photovoltaic power increases or decreases. Thissystem can include the combination of multiple energy storage devices,multiple power generation devices, or multiple energy storage and powergeneration devices.

The plant control system can further include inertial set points tosuppress the rate of plant output power rate of change whentransitioning to a different power rate change condition including anegative power rate change to a positive power rate change condition ortransitioning from a positive power rate change to negative power ratechange condition. The system can further include using the plantirradiance measurement as an anticipatory indicator in determining whena change in the power output rate change should be made and theapproximate new value of the power rate change limits. The system canfurther include slowing or reducing the plant output rate of change whenthe plant measured irradiance value reaches a relatively stable minimumor maximum value. The system can further include adjusting the rate atwhich stored energy is increased in the stored energy system, up to theallowable positive power rate change limit, if the irradiancemeasurement is increasing. The system can further include adjusting therate at which stored energy is decreased in the stored energy system, upto the allowable negative power rate change limit, if the irradiancemeasurement is decreasing. The system can further include adjusting therate at which auxiliary power is decreased, up to the allowable positivepower rate change limit, if the irradiance measurement is increasing.The system can further include adjusting the rate at which auxiliarypower is increased, up to the allowable negative power rate changelimit, if the irradiance measurement is decreasing.

The plant control system can further include operation of the plant tofollow a predetermined energy/power schedule determined by solar energyproduction forecasting techniques. The energy/power forecast schedulemay predict the energy and average power expected from the plant foreach hour of production. The system can further include an upper powerforecast limit and a lower power forecast limit based on the averageforecasted power within an hour period. The system can further includedecreasing or increasing the auxiliary power or the level of storedenergy in the stored energy system to maintain the plant output rate ofchange within the power output rate change band if the plant outputpower is within the upper and lower power forecast limits. The systemcan further include decreasing the level of stored energy in the storedenergy system to maintain constant plant output power if the plantoutput power approaches the lower power forecast limit. The system canfurther include increasing the level of stored energy in the storedenergy system to maintain constant plant output power if the plantoutput power approaches the upper power forecast limit. The system canfurther include increasing the auxiliary power to maintain constantplant output power if the plant output power approaches the lower powerforecast limit. The system can further include decreasing the level ofstored energy in the stored energy system to maintain constant plantoutput power if the plant output power approaches the upper powerforecast limit. The system can further include limiting the rate ofchange of the combined photovoltaic and auxiliary power source tooperate within the power output rate change band when transitioning fromone scheduled hour to the next scheduled hour. The plant control systemcan further include adjusting auxiliary power or the level of storedenergy in the stored energy system to control the plant output tominimize any revenue penalties due to the net energy deviation fromscheduled based on predetermined criteria associated with utility ratestructures and tariffs.

The plant control system can adjust auxiliary power or the level ofstored energy in anticipation of and to compensate for cloud cover. Theplant control system can increase auxiliary power or the level of storedenergy when cloud cover is forecast or physically detected. The plantcontrol system can adjust auxiliary power or the level of stored energyin the absence of cloud cover. The plant control system can reduceauxiliary power or the level of stored energy when no cloud cover isforecast or physically detected. The plant control system can adjustauxiliary power or a level of stored energy for grid frequencyregulation, ancillary services, or load shifting.

A system for generating less variable output power can include receivinga photovoltaic array, an inverter connected to the photovoltaic array;and an auxiliary power source, where the auxiliary power source producesalternating current output power to a grid. The system can include aplant control system controlling the inverter and auxiliary powersource, measuring the rate of change of power from the photovoltaicarray, and adjusting auxiliary power output. The system can include theoutput power of the combined photovoltaic power plant and auxiliarypower plant set to operate within a given power output rate change bandwhich defines the maximum allowable plant output power rate of change,positive or negative. The system can include separate and independentlyadjustable set points for positive and negative power rate changelimits. The system can include power rate change limits that arecontinuously adjustable between 0% (zero) and 100%, positive andnegative. The system can include set points that can be pre-set and thenautomatically changed by the plant control system in response to time ofday; current, scheduled or anticipated photovoltaic plant operatingconditions; and current or anticipated weather conditions.

The system can include the auxiliary power source being a source that iscontinuously available. The auxiliary power source can be a utilitysupplied energy source. The auxiliary power source can be a source withsignificantly limited energy source or a high rate rechargeable source.The system can include the auxiliary power source being at least one ofa stored energy system and an alternative generation device. The storedenergy system can be one of an electrolytic cell device, a capacitor, alead-acid battery, a nickel-cadmium battery, a sodium battery, alithium-ion battery, a flow battery, or a mechanical kinetic and/orpotential energy storage device. The alternative generation device canbe one of a fuel cell, a wind turbine generator, a flywheel generator, agas fueled prime mover, a liquid fueled prime mover, or a compressed-airpowered prime mover.

The system can include the auxiliary power being a source having thepower and energy capacity and dynamic response appropriate for thesystem configuration and modes of operation as described herein. Sincethe potential power output rate of change of a photovoltaic plant can berelatively rapid, the auxiliary source can have the capability torespond faster. The auxiliary power can be a continuously availableutility supplied energy source. The auxiliary power can be a high raterechargeable source with significantly limited energy storage. Theauxiliary power can be a low rate rechargeable auxiliary power sourcewith significantly large energy storage. Energy storage systems mayinclude mechanical and/or electrical devices for converting and storingenergy. The stored energy system can be, but is not limited to, one ofan electrolytic cell device a capacitor, a lead-acid battery, anickel-cadmium battery, a sodium battery, a lithium-ion battery, a flowbattery, or a mechanical kinetic and/or potential energy storage device.The auxiliary power can be an alternative generation device. Thealternative generation device can be, but is not limited to, one of afuel cell, a wind turbine generator, a flywheel generator, a gas fueledcombustion prime mover-generator, a liquid fueled prime mover-generator,a compressed-gas powered prime mover-generator, or combinations thereof,including an air-powered prime mover-generator. The system can includethe auxiliary power being at least one of a stored energy system and analternative generation device. This system can include the optimumcombination of multiple energy storage device types, multiple powergeneration device types, or multiple energy storage and power generationdevices.

The system can include an auxiliary power source comprising of an energystorage system. The plant control system can increase the level ofstored energy in the auxiliary power source when the photovoltaic outputpower rate of change exceeds the positive limit of the power output ratechange band. The system can decrease photovoltaic power when thephotovoltaic output power rate of change exceeds the positive limit ofthe power output rate change band. The plant control system can maintaina constant level of stored energy in the auxiliary power source when thephotovoltaic output power rate of change is within the positive andnegative limits of the power output rate change band. The plant controlsystem can decrease the level of stored energy in the auxiliary powersource when the photovoltaic output power rate of change is below thenegative limit of the power output rate change band. The system caninclude secondary positive and negative power rate change limits whichhave a tighter tolerance than the normal power rate change limits. Thesystem can include set points defining the upper and lower capacitylimits of the stored energy in the auxiliary power source used todetermine the transition from the normal to the secondary power ratechange limits. The system can include upper and lower capacityhysteresis limits of the stored energy in the auxiliary power sourceused to determine the transition from the secondary to the normal powerrate change limits. The plant control system can switch to the secondarypositive power rate change limit when the level of stored energy fallsbelow the lower capacity limit to facilitate faster charging of theauxiliary power source when the output power rate of change is positive.The plant control system can return to the normal positive power ratechange limit when the level of stored energy in the auxiliary powersource rises above the lower capacity hysteresis limit. The plantcontrol system can switch to the secondary negative power rate changelimit when the level of stored energy rises above the upper capacitylimit to facilitate faster discharging of the auxiliary power sourcewhen the output power rate of change is negative. The plant controlsystem can return to the normal negative power change rate limit whenthe level of stored energy in the auxiliary power source falls below theupper capacity hysteresis limit.

The system can include a plant control system controlling the plantoutput power of the combined photovoltaic array plant and auxiliarypower source to operate at a constant power level. The system caninclude separate and independently adjustable set points for positiveand negative power rate change limits. The system can include set pointslimiting the plant power rate change that are continuously adjustablebetween 0% (zero) and 100%, positive and negative. The plant controlsystem can operate the plant to maintain constant power equal to adispatch set point. The system can operate the plant to maintain aconstant dispatch set point. The system can operate the plant to followa schedule consisting of a series of different dispatch set points. Thesystem can increase the level of stored energy in the auxiliary powersource when the photovoltaic plant output is greater than the dispatchset point. The system can lower the level of stored energy in theauxiliary power source when the photovoltaic plant output is less thanthe dispatch set point. The system can limit the rate of change of thecombined photovoltaic power and auxiliary power source to operate withinthe power output rate change band when the dispatch set point ischanged.

The system can include an auxiliary power source comprising of a powergeneration device. The plant control system can reduce the power outputfrom the auxiliary power source when the photovoltaic power rate ofchange exceeds the positive limit of the power output rate change band.The plant control system can maintain a constant power output from theauxiliary power source when the photovoltaic power rate of change iswithin the positive and negative limits of the power output rate changeband. The plant control system can increase the power output from theauxiliary power source when the photovoltaic power rate of change isbelow the negative limit of the power output rate change band. The plantcontrol system can decrease photovoltaic power when a photovoltaicoutput power rate of change exceeds a positive limit of the power outputrate change band. The plant control system can maintain output power ata constant level if photovoltaic power increases or decreases. Thissystem can include the combination of multiple energy storage devices,multiple power generation devices, or multiple energy storage and powergeneration devices.

The plant control system can further include inertial set points tosuppress the rate of plant output power rate of change whentransitioning to a different power rate change condition including anegative power rate change to a positive power rate change condition ortransitioning from a positive power rate change to negative power ratechange condition. The system can further include using the plantirradiance measurement as an anticipatory indicator in determining whena change in the power output rate change should be made and theapproximate new value of the power rate change limits. The system canfurther include slowing or reducing the plant output rate of change whenthe plant measured irradiance value reaches a relatively stable minimumor maximum value. The system can further include adjusting the rate atwhich stored energy is increased in the stored energy system, up to theallowable positive power rate change limit, if the irradiancemeasurement is increasing. The system can further include adjusting therate at which stored energy is decreased in the stored energy system, upto the allowable negative power rate change limit, if the irradiancemeasurement is decreasing. The system can further include adjusting therate at which auxiliary power is decreased, up to the allowable positivepower rate change limit, if the irradiance measurement is increasing.The system can further include adjusting the rate at which auxiliarypower is increased, up to the allowable negative power rate changelimit, if the irradiance measurement is decreasing.

The plant control system can further include operation of the plant tofollow a predetermined energy/power schedule determined by solar energyproduction forecasting techniques. The energy/power forecast schedulemay predict the energy and average power expected from the plant foreach hour of production. The system can further include an upper powerforecast limit and a lower power forecast limit based on the averageforecasted power within an hour period. The system can further includedecreasing or increasing the auxiliary power or the level of storedenergy in the stored energy system to maintain the plant output rate ofchange within the power output rate change band if the plant outputpower is within the upper and lower power forecast limits. The systemcan further include decreasing the level of stored energy in the storedenergy system to maintain constant plant output power if the plantoutput power approaches the lower power forecast limit. The system canfurther include increasing the level of stored energy in the storedenergy system to maintain constant plant output power if the plantoutput power approaches the upper power forecast limit. The system canfurther include increasing the auxiliary power to maintain constantplant output power if the plant output power approaches the lower powerforecast limit. The system can further include decreasing the level ofstored energy in the stored energy system to maintain constant plantoutput power if the plant output power approaches the upper powerforecast limit. The system can further include limiting the rate ofchange of the combined photovoltaic and auxiliary power source tooperate within the power output rate change band when transitioning fromone scheduled hour to the next scheduled hour. The plant control systemcan further include adjusting auxiliary power or the level of storedenergy in the stored energy system to control the plant output tominimize any revenue penalties due to the net energy deviation fromscheduled based on predetermined criteria associated with utility ratestructures and tariffs.

The plant control system can adjust auxiliary power or the level ofstored energy in anticipation of and to compensate for changing cloudcover. The plant control system can increase auxiliary power or thelevel of stored energy when cloud cover is forecast or physicallydetected. The plant control system can adjust auxiliary power or thelevel of stored energy in the absence of cloud cover. The plant controlsystem can reduce auxiliary power or the level of stored energy when nocloud cover is forecast or physically detected. The plant control systemcan adjust auxiliary power or the level of stored energy for gridfrequency regulation, ancillary services, or peak load shifting whileproducing less variable output from a large, grid connected photovoltaicplant.

The auxiliary power source can be various types ranging from a sourcethat is continuously available with a utility supplied energy source toa source with significantly limited energy source including high raterechargeable sources. Since the potential power output rate of change ofa photovoltaic plant can be relatively rapid, the auxiliary source canhave the capability to respond faster.

Referring now to FIG. 2, the system has a defined algorithm. As shown inFIG. 2, the system 200 includes photovoltaic arrays 205 connected to aplant control system 210. The plant control system 210 includes a DC/DCconverter 215 and an inverter 220. The photovoltaic arrays 205 areconnected to the inverter 220 and then to the grid 225. Additionally, abattery 230 is connected to the DC/DC converter 215. The battery 230, asthe auxiliary power source, is used to reduce the variability of thephotovoltaic power plant output. The battery 230 may be one of thestored energy devices previously described. The battery 230 is normallymaintained at approximately 50% state of charge. Normally, the inverter220, operating to control the maximum power point of the photovoltaicarray 205, delivers power from the photovoltaic array 205 to the grid225 based on the available irradiance. The power level increases anddecreases continuously over the course of a day as available sunlightchanges. The normal absolute value (ABS) rate of change over a day isless than 0.5% of peak rated power per minute.

When solar variability causes the rate of change in photovoltaic plantoutput power to exceed the limits of a normal power output rate changeband, e.g., faster than 3% per minute, positive or negative, thephotovoltaic plant control system 210 controls the DC/DC converter 215to either supply current from or to the battery 230 to control thecurrent into the inverter 220 to maintain the change in power deliveredto the grid 225 within the normal positive and negative limits of thepower output rate change band. The power output rate change band isdefined by separate and independently changeable positive and negativepower rate change limits. The negative power rate change limit may beany value between 0 and −100% per minute. The positive power rate changelimit may be any value between and 0 and +100% per minute. The powerrate change limits may be pre-set by the user and may be automaticallyadjusted by the plant control system 210 based on local variabilitylimitations, current or scheduled plant operating conditions, time ofday, or current or forecasted weather conditions. Other user-definedpre-sets are included to set boundaries on power levels, stored energycontrol, and auxiliary power source operation, as explained herein. Forinstance, when photovoltaic power output decreases at a rate faster thanthe negative power rate change limit, energy stored in the battery 230is discharged by controlling the DC/DC converter current to flow fromthe battery 230 to the inverter 220. This action adds the batterycurrent to the photovoltaic current, providing sufficient current to theinverter 220, reducing the rate of change of the plant output power tobe equal to the negative power rate change limit. In this mode, thebattery 230 is discharged and energy is lost. The photovoltaic plantcontrol system 210 continuously monitors grid output power and adjuststhe DC/DC converter current level and direction, when necessary, so thatgrid output power is within the positive and negative limits of thepower output rate change band. When the photovoltaic output rate ofchange is within the positive and negative limits of the power ratechange band, the DC/DC converter current is reduced ultimately to zero.Similarly, if the photovoltaic power output increases at a rate fasterthan the positive power rate change limit, a portion of the current fromthe photovoltaic array 205 is delivered to the battery 230 bycontrolling the DC/DC converter current to flow into the battery 230.This action subtracts the battery current from the photovoltaic current,reducing the rate of change of the plant output power to be equal to thepositive power rate change limit. In this mode, the battery 230 ischarged and energy is stored. If the energy released and absorbed overtime is equal, the overall state of charge of the battery 230 will bereduced only by the charging and discharging losses.

In systems that employ a limited energy battery 230 (i.e. rated powerfor less than 60 minutes), the plant control system 210 monitors thestate of charge (SOC) of the battery 230. The plant control system 210adjusts the positive power rate change limit when the SOC reaches auser-defined lower capacity limit. The plant control system 210 adjuststhe negative power rate change limit when the SOC reaches a user definedupper capacity limit. If the battery 230 reaches the upper capacitylimit, the negative power rate change limit is reduced to facilitate aquicker discharge rate during negative power rate change conditions,preventing the battery 230 from reaching maximum capacity. The plantcontrol system 210 contains a programmable hysteresis for the uppercapacity limit. The upper capacity hysteresis limit is included toprevent rapid changing of the negative power rate change limit after theupper capacity limit has been reached. For the plant control system 210to return the negative power rate change limit to its normal value, theSOC must drop below the upper capacity hysteresis limit. If the battery230 reaches its maximum SOC, the plant control system 210 will controlthe inverter 220 to reduce the plant output rate of change by curtailingphotovoltaic power. If the battery 230 reaches the lower capacity limit,the positive power rate change limit is decreased to facilitate aquicker charge rate during positive power rate change conditions,preventing the battery 230 from reaching its zero capacity level. Theplant control system 210 contains a programmable hysteresis for thelower capacity limit. The lower capacity hysteresis limit is included toprevent rapid changing of the positive power rate change limit after thelower capacity limit has been reached. For the plant control system 210to return the positive rate change limit to its normal value, the SOCmust rise above the lower capacity hysteresis limit. The control system210 continuously monitors grid output power and battery SOC, andautomatically adjusts battery power level, power flow direction, andrate of change, if necessary, so that the grid output power rate ofchange is contained within the power output rate change band and batteryavailability is maximized. When the output rate of change is within thepower output rate change band, the battery power is reduced to zero.When the battery SOC is between the upper and lower capacity limits, thepositive and negative power rate change limits are set to their normalvalues.

The plant control system 210 also monitors the transition between poweroutput rate change conditions. The plant control system 210 may containan inertia set point. The purpose of this programmed inertia is tosuppress the rate of the power output change rate when the planttransitions to a different power rate change condition including thetransition from a negative power rate change to a positive power ratechange or the transition from a positive power rate change to a negativepower rate change. The plant control system 210 may use the measurementfrom the irradiance sensor 250 to serve as an anticipatory indicator indetermining when a change in the power rate change limits should be madeand the approximate new value of the power rate change limits. If theirradiance measurement reaches a stable minimum or maximum value, theplant control system 210 can slow the plant output rate of change tonear 0% per minute. If the irradiance sensor 250 indicates an increasingirradiance level, the plant control system 210 will control the DC/DCconverter 215 to adjust the rate at which the battery 230 is charging,up to the allowable positive power rate change limit. If the irradiancesensor 250 indicates a decreasing irradiance level, the plant controlsystem 210 will control the DC/DC converter 215 to adjust the rate atwhich the battery 230 is discharging, up to the allowable negative powerrate change limit.

Additional plant control input is received from weather forecastingservice data 245 and from ground based solar radiation sensors 250.Sensors 250 on the ground at the photovoltaic array site 205 and sensors250 located within an appropriate distance from the photovoltaic plantin the general direction of prevailing winds and weather are used todetermine impending cloud cover in order to manage plant output and theenergy storage system SOC. Should an impending change in cloud cover bedetected by sensors 250, the plant control system 210 may anticipate theeffect on photovoltaic plant irradiance level and adjust the SOC of thebattery 230 either higher or lower, as needed, by either charging ordischarging the battery 230 to minimize plant output variability.Whenever the control system 210 determines that the energy storagesystem will not be needed to reduce photovoltaic power variability basedon weather forecast data, it shall make the battery 230 available forgrid frequency regulation, other revenue producing ancillary services,and peak load shifting, depending upon auxiliary source capacity.

The weather forecasting service data 245 can also provide apredetermined energy/power schedule determined by solar energyproduction forecasting techniques. The energy/power forecast schedulemay predict the energy and average power expected from the plant foreach hour of production. Based on the forecast received from the weatherforecasting service data 245, the control system 210 can set an upperpower forecast limit and a lower power forecast limit based on theaverage forecasted power within an hour period. Within each hour period,the plant control system 210 monitors the plant output power. If theplant output power is within the upper and lower power forecast limitsfor the hour, the plant control system 210 will operate the plant tomaintain the rate of change of the plant output to be within thepositive and negative limits of the power output rate change banddescribed herein. If the upper power forecast limit is reached withinthe hour, the plant control system 210 will control the DC/DC converter215 to charge the battery 230 to maintain constant power at the plantoutput equal to the upper power forecast limit. If the lower powerforecast limit is reached within the hour, the plant control system 210will control the DC/DC converter 215 to discharge the battery 230 tomaintain constant power at the plant output equal to the lower powerforecast limit. When changing scheduling intervals, the plant controlsystem 210 will control the DC/DC converter 215 to maintain the rate ofchange of the plant output power to be within the positive and negativelimits of the power output rate change band. The goal of incorporatingthe weather forecasting service data 245 is to minimize any revenuepenalties due to the net energy deviation from scheduled energyproduction based on predetermined criteria associated with utility ratestructures and tariffs.

In systems that employ higher capacity stored energy sources (i.e. ratedpower for more than 60 minutes), the photovoltaic plant controlalgorithm for reducing variability of output power is adaptive so thatbattery rate of charge and discharge, battery state of charge (SOC), andbattery energy in/out, averaged over a rolling period of time, is usedto adjust the positive and negative limits of the power output ratechange band. The algorithm optimizes energy storage capacity by reducingthe normal plant output as variability, magnitude, and duty cycleincreases and increasing normal plant output power as variability,magnitude, and duty cycle decreases. The goal of this algorithm is toreplace energy discharged plus losses from the photovoltaic array 205over time. If the power available to the inverter 220 is greater thanthe maximum capability of the inverter 220, evidenced by an operatingmode of the inverter 220 known as clipping or current limiting, theplant control system 210 will control the DC/DC converter 215 to chargethe battery 230 at a rate that permits the inverter 220 to delivermaximum power to the grid 225 without clipping or current limiting. Thecontrol system 210 continuously monitors grid output power and batterySOC and automatically adjusts battery power level, power flow direction,and rate of change, if necessary, so that grid output power rate ofchange is within the positive and negative limits of the power outputrate change band and battery availability is maximized. When the plantoutput rate of change is within the power rate change limits, thebattery power is reduced to zero. When the battery SOC is between highand low levels, the positive and negative power rate change limits areset to their normal values. A one day operational cycle of thisalgorithm is shown in FIG. 6.

In systems that employ higher capacity stored energy sources (i.e. ratedpower for more than 60 minutes), the photovoltaic plant controlalgorithm for reducing variability of output power can be a constantdispatch signal. The dispatch signal shall be defined by the plantcontrol system 210. The algorithm optimizes energy storage capacity byallowing the normal plant output energy to be delivered coincident withthe peak utility demand and the highest time of day (TOD) rates. Throughthis method of control, the plant outputs a constant power level inaccordance with the dispatch set point. The plant output is restrictedto operate within the power output rate change band described above. Ifthe dispatch level is changed, the plant output power will be regulatednot to exceed the positive or negative power rate change limits. Thegoal of this algorithm is to provide constant power output, maximizeeconomic dispatch and recover energy discharged plus losses from thephotovoltaic array 205 over time. If the power available to the inverter220 is greater than the dispatch set point, the plant control system 210will control the DC/DC converter 215 to charge the battery 230 at a ratethat permits the inverter 220 to deliver constant power to the grid 225.If the power available to the inverter 220 is less than the dispatch setpoint, the plant control system 210 will control the DC/DC converter 215to discharge the battery 230 at a rate that permits the inverter 220 todeliver constant power to the grid 225. The control system 210continuously monitors grid output power and battery SOC andautomatically adjusts battery power level, power flow direction so thatgrid output power is constant in accordance with the dispatch set point.If the plant control system 210 is commanded to change its dispatch setpoint, the plant control system shall raise or lower the output power ata rate that maintains plant output within the limits of power outputrate change band.

Referring now to FIG. 3, the system has a defined algorithm. As shown inFIG. 3, the system 300 includes photovoltaic arrays 305 connected to aplant control system 310. The plant control system 310 includes abattery inverter 315 and a photovoltaic inverter 320. The photovoltaicarrays 305 are connected to the photovoltaic inverter 320 and then tothe grid 325. Additionally, a battery 330 may be one of the storedenergy devices previously described. The battery 330 is normallymaintained at approximately 50% state of charge. Connected to thebattery inverter 315, the battery 330 is used to reduce the variabilityof the photovoltaic power plant output. Unlike in the system of FIG. 2,however, the battery inverter 320 is connected at the AC or grid side ofthe inverter 320.

When the rate of change in photovoltaic plant output power exceeds thelimits of a power output rate change band, e.g., faster than 3% perminute, positive or negative, the plant control system 310 controls thebattery inverter 315 to either supply power from or to the battery 330to control the change in power delivered to the grid 325 within thenormal positive and negative limits of the power output rate changeband. The power output rate change band is defined by separate andindependently changeable positive and negative power rate change limits.The negative power rate change limit may be any value between 0 and−100% per minute. The positive power rate change limit may be any valuebetween and 0 and +100% per minute. The power rate change limits may bepre-set by the user and may be automatically adjusted by the plantcontrol system 310 based on local variability limitations, current orscheduled plant operating conditions, time of day, or current orforecasted weather conditions. Other user-defined pre-sets are includedto set boundaries on power levels, stored energy control, and auxiliarypower source operation, as explained herein. For instance, whenphotovoltaic power decreases at a rate faster than the negative powerrate change limit, energy stored in the battery 330 is discharged bycontrolling battery inverter power to flow from the battery 330 to thegrid 325. This action adds the battery power to the photovoltaic plantoutput power, reducing the rate of change of the plant power output tobe equal to the negative power rate change limit. In this mode, thebattery 330 is discharged and energy is lost. The photovoltaic plantcontrol system 310 continuously monitors grid output power and adjuststhe battery inverter power level and direction, when necessary, so thatgrid output power is within the positive and negative limits of thepower output rate change band. When the photovoltaic output rate ofchange is within the positive and negative limits of the power outputrate change band, the battery inverter current is reduced ultimately tozero. Similarly, if photovoltaic output power increases at a rate fasterthan the positive power rate change limit, a portion of the photovoltaicplant output power is delivered to the battery 330 by controlling thebattery inverter power to flow into the battery 330. This actionsubtracts the battery power from the photovoltaic plant output power,reducing the rate of change of the plant output power to be equal to thepositive power rate change limit. In this mode the battery 330 ischarged and energy is stored. If the energy released and absorbed overtime is equal, the overall state of charge of the battery will reducedonly by the charging and discharging losses.

In systems that employ a limited energy battery 330 (i.e. rated powerfor less than 60 minutes), the control system 310 monitors the state ofcharge (SOC) of the battery 330. The plant control system 310 adjuststhe positive power rate change limit when the SOC reaches a user-definedlower capacity limit. The plant control system 310 adjusts the negativepower rate change limit when the SOC reaches a user defined uppercapacity limit. If the battery 330 reaches the upper capacity limit, thenegative power rate change limit is reduced to facilitate a quickerdischarge rate during negative power rate change conditions, preventingthe battery 330 from reaching maximum capacity. The plant control system310 contains a programmable hysteresis for the upper capacity limit. Theupper capacity hysteresis limit is included to prevent rapid changing ofthe negative power rate change limit after the upper capacity limit hasbeen reached. For the plant control system 310 to return the negativepower rate change limit to its normal value, the SOC must drop below theupper capacity hysteresis limit. If the battery 330 reaches its maximumSOC, the plant control system 310 will control the inverter 320 toreduce the plant output rate of change by curtailing photovoltaic power.If the battery 330 reaches the lower capacity limit, the positive powerrate change limit is decreased to facilitate a quicker charge rateduring positive power rate change conditions, preventing the battery 330from reaching its zero capacity level. The plant control system 310contains a programmable hysteresis for the lower capacity limit. Thelower capacity hysteresis limit is included to prevent rapid changing ofthe positive power rate change limit after the lower capacity limit hasbeen reached. For the plant control system 310 to return the positivepower rate change limit to its normal value, the SOC must rise above thelower capacity hysteresis limit. The control system 310 continuouslymonitors grid output power and battery SOC and automatically adjustsbattery power level, power flow direction, and rate of change, ifnecessary, so that grid output power rate of change is within theoperating band and battery availability is maximized. When the outputrate of change is within the power output rate change band, the batterypower is reduced to zero. When the battery SOC is between the upper andlower capacity limits, the positive and negative power rate changelimits are set to their normal values. A day's operational cycle of thisalgorithm is shown in FIG. 7.

The plant control system 310 also monitors the transition between poweroutput rate change conditions. The plant control system 310 may containan inertia set point. The purpose of this programmed inertia is tosuppress the rate of the power output change rate when the planttransitions to a different power rate change condition including thetransition from a negative power rate change to a positive power ratechange or the transition from a positive power rate change to a negativepower rate change. The plant control system 310 may use the measurementfrom the irradiance sensor 350 to serve as an anticipatory indicator indetermining when a change in the power rate change limits should be madeand the approximate new value of the power rate change limits. If theirradiance measurement reaches a stable minimum or maximum value, theplant control system 310 can slow the plant output rate of change tonear 0% per minute. If the irradiance sensor 350 indicates an increasingirradiance level, the plant control system 310 will control the batteryinverter 315 to adjust the rate at which the battery 330 is charging, upto the allowable positive power rate change limit. If the irradiancesensor 350 indicates a decreasing irradiance level, the plant controlsystem 310 will control the battery inverter 315 to adjust the rate atwhich the battery 330 is discharging, up to the allowable negative powerrate change limit.

Additional plant control input is received from weather forecastingservice data 345 and from ground based solar radiation sensors 350.Sensors 350 on the ground at the photovoltaic array site 305 and sensors350 located within an appropriate distance from the photovoltaic plantin the general direction of prevailing winds and weather are used todetermine impending cloud cover in order to manage plant output and theenergy storage system SOC. Should an impending change in cloud cover bedetected by sensors 350, the plant control system 310 may anticipate theeffect on photovoltaic plant irradiance level and adjust the SOC of thebattery 330 either higher or lower, as needed, by either charging ordischarging the battery 330 to minimize plant output variability.Whenever the control system 310 determines that the energy storagesystem will not be needed to reduce photovoltaic power variability basedon weather forecast data, it shall make the battery 330 available forgrid frequency regulation, other revenue producing ancillary services,and peak load shifting, depending upon auxiliary source capacity.

The weather forecasting service data 345 can also provide apredetermined energy/power schedule determined by solar energyproduction forecasting techniques. The energy/power forecast schedulemay predict the energy and average power expected from the plant foreach hour of production. Based on the forecast received from the weatherforecasting service data 345, the control system 310 can set an upperpower forecast limit and a lower power forecast limit based on theaverage forecasted power within an hour period. Within each hour period,the plant control system 310 monitors the plant output power. If theplant output power is within the upper and lower power forecast limitsfor the hour, the plant control system 310 will operate the plant tomaintain the rate of change of the plant output to be within thepositive and negative limits of the power output rate change banddescribed herein. If the upper power forecast limit is reached withinthe hour, the plant control system 310 will control the battery inverter315 to charge the battery 330 to maintain constant power at the plantoutput equal to the upper power forecast limit. If the lower powerforecast limit is reached within the hour, the plant control system 310will control the battery inverter 315 to discharge the battery 330 tomaintain constant power at the plant output equal to the lower powerforecast limit. When changing scheduling intervals, the plant controlsystem 310 will control the battery inverter 315 to maintain the rate ofchange of the plant output power to be within the positive and negativelimits of the power output rate change band. The goal of incorporatingthe weather forecasting service data 345 is to minimize any revenuepenalties due to the net energy deviation from scheduled energyproduction based on predetermined criteria associated with utility ratestructures and tariffs.

In systems that employ higher capacity stored energy sources (i.e. ratedpower for more than 60 minutes), the photovoltaic plant controlalgorithm for reducing variability of output power is adaptive so thatbattery rate of charge and discharge, battery state of charge (SOC), andbattery energy in/out, averaged over a rolling period of time, is usedto adjust the positive and negative limits of the power output ratechange band. The algorithm optimizes energy storage capacity by reducingthe normal plant output as variability, magnitude, and duty cycleincreases and increasing normal plant output power as variability,magnitude, and duty cycle decreases. The goal of this algorithm is toreplace energy discharged plus losses from the photovoltaic array overtime.

In systems that employ higher capacity stored energy sources (i.e. ratedpower for more than 60 minutes), the photovoltaic plant controlalgorithm for reducing variability of output power can be a constantdispatch signal. The dispatch signal shall be defined by the plantcontrol system 310. The algorithm optimizes energy storage capacity byallowing the normal plant output energy to be delivered coincident withthe peak utility demand and with the highest time of day (TOD) rates.Through this method of control, the plant outputs a constant power levelin accordance with the dispatch set point. The plant output isrestricted to operate within the power output rate change band describedabove. If the dispatch level is changed, the plant output power will beregulated not to exceed the positive or negative power rate changelimits. The goal of this algorithm is to provide constant power output,maximize economic dispatch and recover energy discharged plus lossesfrom the photovoltaic array 305 over time. If the power delivered by thephotovoltaic inverter 320 is greater than the dispatch set point, theplant control system 310 will control the battery inverter 315 to chargethe battery 330 at a rate that permits the system to provide constantpower to the grid 325. If the power delivered by the photovoltaicinverter 320 is less than the dispatch set point, the plant controlsystem 310 will control the battery inverter 315 to discharge thebattery 330 at a rate that permits the system to deliver constant powerto the grid 325. The control system 310 continuously monitors gridoutput power and battery SOC and automatically adjusts battery powerlevel, power flow direction so that grid output power is constant inaccordance with the dispatch set point. If the plant control system 310is commanded to change its dispatch set point, the plant control systemshall raise or lower the output power at a rate that maintains the plantoutput within the limits of the power output rate change band.

Referring now to FIG. 4, the system has a defined algorithm. As shown inFIG. 4, the system 400 includes photovoltaic arrays 405 connected to aplant control system 410. The plant control system 410 includes agenerator 415 and a photovoltaic inverter 420. The photovoltaic arrays405 are connected to the photovoltaic inverter 420 and then to the grid425. The generator 415 may be one of the types previously described. Thegenerator 415 is connected to the grid 425 and normally operated at aload level that permits it to be increased or decreased by an amountequal to approximately 30 to 40% of the photovoltaic plant rated ACoutput. A 10 MW rated photovoltaic plant would need a generator plantcapable of 10 MW peak power (non-continuous) and normally would beoperating at a load point equal to 65% of the photovoltaic plant outputover the course of an operating day. The generator 415 is used to reducethe variability of the photovoltaic power plant output. Like in thesystem of FIG. 3, the generator 415 is connected at the AC or grid sideof the photovoltaic inverter 420.

When the rate of change of the photovoltaic plant output power exceedsthe limits of the power output rate change band, e.g., faster than 3%per minute, positive or negative, the plant control system 410 controlsthe generator 415 to either decrease or increase power to control thechange in power delivered to the grid 425. As above, the power outputrate change band is defined by separate and independently changeablepositive and negative power rate change limits. The negative power ratechange limit may be any value between 0 and −100% per minute. Thepositive power rate change limit may be any value between and 0 and+100% per minute. The power rate change limits may be pre-set by theuser and may be automatically adjusted by the plant control system 410based on local variability limitations, current or scheduled plantoperating conditions, time of day, or current or forecasted weatherconditions. Other user-defined pre-sets are included to set boundarieson power levels and auxiliary power source operation, as explainedherein. For instance, when photovoltaic power decreases at a rate fasterthan the negative power rate change limit, the generator power isincreased to the grid 425. This action adds to photovoltaic plant outputpower, reducing the rate of change of the plant output power to be equalto the negative power change rate limit. The photovoltaic plant controlsystem 410 continuously monitors grid output power and adjusts thegenerator power level, when necessary, so that grid output power iswithin the positive and negative limits of the power output rate changeband. When photovoltaic output rate of change is within the positive andnegative limits of the power output rate change band, the generatorpower is set to its normal operating load point. Similarly, ifphotovoltaic output power increases faster than positive power ratechange limit, generator power is decreased. This action subtracts powerfrom photovoltaic plant output power, reducing the rate of change of theplant output power to be equal to the positive power rate change limit.

The plant control system 410 also monitors the transition between poweroutput rate change conditions. The plant control system 410 may containan inertia set point. The purpose of this programmed inertia is tosuppress the rate of the power output change rate when the planttransitions to a different power rate change condition including thetransition from a negative power rate change to a positive power ratechange or the transition from a positive power rate change to a negativepower rate change. The plant control system 410 may use the measurementfrom the irradiance sensor 450 to serve as an anticipatory indicator indetermining when a change in the power rate change limits should be madeand the approximate new value of the power rate change limits. If theirradiance measurement reaches a stable minimum or maximum value, theplant control system 410 can slow the plant output rate of change tonear 0% per minute. If the irradiance sensor 450 indicates an increasingirradiance level, the plant control system 410 will adjust the rate atwhich the generator 415 reduces its output power, up to the allowablepositive power rate change limit. If the irradiance sensor 450 indicatesa decreasing irradiance level, the plant control system 410 will adjustthe rate at which the generator 415 increases its output power, up tothe allowable negative power rate change limit.

Additional plant control input is received from weather forecastingservice data 445 and from ground based solar radiation sensors 450.Sensors 450 on the ground at the photovoltaic array site 405 and sensors450 located within an appropriate distance from the photovoltaic plantin the general direction of prevailing winds and weather are used todetermine impending cloud cover in order to manage plant output and thebase load power output from the generator 415. Should an impendingchange in cloud cover be detected by sensors 450, the plant controlsystem 410 may anticipate the effect on photovoltaic plant irradiancelevel and adjust the power output of the generator 415 either higher orlower, as needed to minimize plant output variability.

The weather forecasting service data 445 can also provide apredetermined energy/power schedule determined by solar energyproduction forecasting techniques. The energy/power forecast schedulemay predict the energy and average power expected from the plant foreach hour of production. Based on the forecast received from the weatherforecasting service data 445, the control system 410 can set an upperpower forecast limit and a lower power forecast limit based on theaverage forecasted power within an hour period. Within each hour period,the plant control system 410 monitors the plant output power. If theplant output power is within the upper and lower power forecast limitsfor the hour, the plant control system 410 will operate the plant tomaintain the rate of change of the plant output to be within thepositive and negative limits of the power output rate change banddescribed herein. If the upper power forecast limit is reached withinthe hour, the plant control system 410 will command the generator 415 toreduce its output power and maintain constant power at the plant outputequal to the upper power forecast limit. If the lower power forecastlimit is reached within the hour, the plant control system 410 willcommand the generator 415 to increase its output and maintain constantpower at the plant output equal to the lower power forecast limit. Whenchanging scheduling intervals, the plant control system 410 will controlthe generator 415 to maintain the rate of change of the plant outputpower to be within the positive and negative limits of the power outputrate change band. The goal of incorporating the weather forecastingservice data 445 is to minimize any revenue penalties due to the netenergy deviation from scheduled energy production based on predeterminedcriteria associated with utility rate structures and tariffs.

Referring now to FIG. 5, the system has a defined algorithm. As shown inFIG. 5, the system 500 includes photovoltaic arrays 505 connected to aplant control system 510. The plant control system 510 includes a DC/DCconverter 515 and an inverter 520. The photovoltaic arrays 505 areconnected to the inverter 520 and then to the grid 525. Additionally, abattery 530 and fuel cell 535 are connected to the DC/DC converter 515.The combination of the battery 530 and fuel cell 535 as the auxiliarypower source is used to reduce the variability of the photovoltaic powerplant output. The battery 530 may be one of the stored energy devicespreviously described and is included in the system to provide rapidresponse. The fuel cell 535 is intended to provide bulk energy. Thebattery 530 is normally maintained at approximately 50% state of charge.Additionally, similar to the systems described above, additionalinformation is received from weather forecasting service data 545 andfrom ground based solar radiation sensors 550. Similar to the system 400shown in FIG. 4, the fuel cell 535 is normally operated at a load levelthat permits it to be increased or decreased by an amount equal toapproximately 30 to 40% of the photovoltaic plant rated AC output. A 10MW rated photovoltaic plant would need a fuel cell plant capable of 10MW peak power (non-continuous) and normally would be operating at a loadpoint equal to 65% of photovoltaic plant output over the course of anoperating day.

When solar variability causes the rate of change of the photovoltaicplant output power to exceed the limits of the power output rate changeband, e.g., faster than 3% per minute, positive or negative, thephotovoltaic plant control system 510 controls the DC/DC converter 515to either supply current from or to the battery 530 to control thecurrent into the inverter 520 to maintain the change in power deliveredto the grid 525 within the positive and negative power rate changelimits. Concurrently, the plant control system 510 will increase ordecrease the output of the fuel cell 535 to follow the action of thebattery 530. In this way the system operates similarly to the one shownin FIG. 4. The system 500 can maintain separate and independentlychangeable positive and negative power rate change limits. The negativepower rate change limit of the band may be any value between 0 and −100%per minute. The positive power rate change limit may be any valuebetween and 0 and +100% per minute. The system 500 can further includeset points that can be automatically changed by the plant control systemin response to photovoltaic power plant operating conditions and weatherconditions. The system 500 can also include inertial set points tosuppress the rate of the power output change rate when transitioningbetween different power rate change conditions.

In general, a photovoltaic system is comprised of several modules. Amodule is comprised of two or more submodules connected in parallel. Asubmodule is comprised of series-connected individual cells.Photovoltaic modules can be used in arrays of many, interconnectedmodules.

A common photovoltaic cell can have multiple layers. The multiple layerscan include a bottom layer that is a transparent conductive layer, acapping layer, a window layer, an absorber layer and a top layer. Eachlayer can be deposited at a different deposition station of amanufacturing line with a separate deposition gas supply and avacuum-sealed deposition chamber at each station as required. Thesubstrate can be transferred from deposition station to depositionstation via a rolling conveyor until all of the desired layers aredeposited. A top substrate layer can be placed on top of the top layerto form a sandwich and complete the photovoltaic cell.

The total output current of the module is the sum of the currents ofeach of the sub-modules. Thus, the optimum design of sub-modules withina module is determined by system requirements. In general, photovoltaicmodules are formed by the deposition of multiple semiconductor ororganic thin films on rigid or flexible substrates or superstrates. Theterm superstrate is generally used if the light incident on a modulepasses through the transparent substrate used for semiconductor ororganic film deposition. Electrical contact to the solar cell materialon the substrate side can be provided by an electrically conductivesubstrate material or an electrically conductive layer between the solarcell material and the substrate such as a transparent conductive layeror a transparent conductive oxide (TCO). For superstrates, electricalcontact on the substrate side of the solar cell material can be providedby patterned metal layers and/or a TCO, for example.

A photovoltaic cell can include a second semiconductor material over thefirst semiconductor material. The first semiconductor material can be aCdS. The second semiconductor material can be a CdTe. The substrate canbe glass. A photovoltaic cell can be part of a submodule, which includesgreater than 50 cells. The submodule can also include greater than 80cells. The submodule can also include greater than 100 cells.

A method of manufacturing a system can include providing a transparentconductive layer on a substrate, contacting a first submodule and asecond submodule with the transparent conductive layer through a sharedcell, the first submodule and second submodule being connected inparallel, the first submodule having an electrical contact regionincluding a first trench pattern, wherein the first trench pattern is apattern of photovoltaic cells connected in series and a last cell in theseries is the shared cell.

A method of forming a photovoltaic structure can include depositing asemiconductor layer over a transparent conductive layer, scribing thesemiconductor layer to form a cell, the cell comprising a semiconductormaterial shared by two parallel connected submodules, and metallizingthe cell.

A method of forming a photovoltaic structure can include depositing asemiconductor layer over a transparent conductive layer, scribing asemiconductor layer to form a cell, placing a metal layer over the celland forming two electrical contacts between the transparent conductivelayer and the metal layer.

In this system, a photovoltaic cell can be constructed of a series oflayers of semiconductor materials deposited on a glass substrate. In anexample of a common photovoltaic cell, the multiple layers can include:a bottom layer that is a transparent conductive layer, a window layer,an absorber layer, and a top layer. The top layer can be a metal layer.Each layer can be deposited at a different deposition station of amanufacturing line with a separate deposition gas supply and avacuum-sealed deposition chamber at each station as required. Thesubstrate can be transferred from deposition station to depositionstation via a rolling conveyor until all of the desired layers aredeposited. Additional layers can be added using other techniques such assputtering. Electrical conductors can be connected to the top and thebottom layers respectively to collect the electrical energy producedwhen solar energy is incident onto the absorber layer. A top substratelayer can be placed on top of the top layer to form a sandwich andcomplete the photovoltaic device.

The bottom layer can be a transparent conductive layer, and can be forexample a transparent conductive oxide such as zinc oxide, zinc oxidedoped with aluminum, tin oxide or tin oxide doped with fluorine.Sputtered aluminum doped zinc oxide has good electrical and opticalproperties, but at temperatures greater than 500° C., aluminum dopedzinc oxide can exhibit chemical instability. In addition, at processingtemperatures greater than 500° C., oxygen and other reactive elementscan diffuse into the transparent conductive oxide, disrupting itselectrical properties.

The window layer and the absorbing layer can include, for example, abinary semiconductor such as group II-VI, III-V or IV semiconductor,such as, for example, ZnO, ZnS, ZnSe, ZnTe, CdO, CdS, CdSe, CdTe, MgO,MgS, MgSe, MgTe, HgO, HgS, HgSe, HgTe, AN, AlP, AlAs, AlSb, GaN, GaP,GaAs, GaSb, InN, InP, InAs, InSb, TlN, TlP, TlAs, TlSb, or mixtures,compounds or alloys thereof. An example of a window layer and absorbinglayer is a layer of CdS coated by a layer of CdTe.

A metal layer can be deposited as an electrical contact to asemiconductor layer for solar device operation, as taught, for example,in U.S. Patent Application Ser. No. 60/868,023, which is herebyincorporated by reference in its entirety. A metal layer can be acomposite layer comprised of metal layers, such as a Cr/Al/Cr metalstack. The metal layers in a composite layer can be metals that have athermal expansion coefficient between the semiconductor layer and afirst metal layer. Metal adhesion is impacted by intrinsic stress, whichis a function of deposition variables. Metal adhesion is also impactedby extrinsic stresses such as post-deposition thermal treatment in whichcase dissimilarity in thermal expansion coefficients may contribute toreduced adhesion. A proper sequential arrangement of metals, such aschromium, nickel, and aluminum, can provide a gradient in thermalexpansion of the metal stack thereby minimizing loss of adhesion duringthermal processing.

Additional metal layers can be added in order to provide a gradient ofthermal expansion coefficients thereby minimizing de-lamination duringheat treatment. Adhesion has been shown to be improved when thermalexpansion coefficients of selected materials were more closely matched.

Additional layers, such as a protective layer of material with a highchemical stability, or a capping layer can also be provided. Cappinglayers are described, for example, in U.S. Patent Publication20050257824, which is incorporated by reference herein.

A method of making a photovoltaic cell can include placing asemiconductor layer on a substrate and depositing a metal layer incontact with a semiconductor layer to metallize a photovoltaic cell. Incertain circumstances a metal layer can be a chromium-containing layer.In other circumstances, metal layers can be deposited sequentially toform a metal stack. For example, a first metal layer can be achromium-containing layer, a third metal layer can be analuminum-containing layer, and second layer between the first and thirdmetal layers can be a nickel-containing layer. In another embodiment, aphotovoltaic device can further comprise a fourth layer, wherein thefourth layer is an intermediate layer between the second metal layer andthe third metal layer. The intermediate layer can be a nickel-containinglayer. A metal layer can also include tungsten, molybdenum, iridium,tantalum, titanium, neodymium, palladium, lead, iron, silver, or nickel.

In certain circumstances, a capping layer can be deposited in additionto a tin oxide protective layer. A capping layer can be positionedbetween the transparent conductive layer and the window layer. Thecapping layer can be positioned between the protective layer and thewindow layer. The capping layer can be positioned between thetransparent conductive layer and the protective layer. The capping layercan serve as a buffer layer, which can allow a thinner window layer tobe used. For example, when using a capping layer and a protective layer,the first semiconductor layer can be thinner than in the absence of thebuffer layer. For example, the first semiconductor layer can have athickness of greater than about 10 nm and less than about 600 nm. Forexample, the first semiconductor layer can have a thickness greater than20 nm, greater than 50 nm, greater than 100 nm, or greater than 200 nmand less than 400 nm, less than 300 nm, less than 250 nm, or less than150 nm.

The first semiconductor layer can serve as a window layer for the secondsemiconductor layer. By being thinner, the first semiconductor layerallows greater penetration of the shorter wavelengths of the incidentlight to the second semiconductor layer. The first semiconductor layercan be a group II-VI, III-V or IV semiconductor, such as, for example,ZnO, ZnS, ZnSe, ZnTe, CdO, CdS, CdSe, CdTe, MgO, MgS, MgSe, MgTe, HgO,HgS, HgSe, HgTe, AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, InN, InP,InAs, InSb, TlN, TlP, TlAs, TlSb, or mixtures, compounds or alloysthereof. It can be a binary semiconductor, for example it can be CdS.The second semiconductor layer can be deposited onto the firstsemiconductor layer. The second semiconductor can serve as an absorberlayer for the incident light when the first semiconductor layer isserving as a window layer. Similar to the first semiconductor layer, thesecond semiconductor layer can also be a group II-VI, III-V or IVsemiconductor, such as, for example, ZnO, ZnS, ZnSe, ZnTe, CdO, CdS,CdSe, CdTe, MgO, MgS, MgSe, MgTe, HgO, HgS, HgSe, HgTe, AlN, AlP, AlAs,AlSb, GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb, TlN, TlP, TlAs, TlSb,or mixtures, compounds or alloys thereof.

Deposition of semiconductor layers in the manufacture of photovoltaicdevices is described, for example, in U.S. Pat. Nos. 5,248,349,5,372,646, 5,470,397, 5,536,333, 5,945,163, 6,037,241, and 6,444,043,each of which is incorporated by reference in its entirety. Thedeposition can involve transport of vapor from a source to a substrate,or sublimation of a solid in a closed system. An apparatus formanufacturing photovoltaic devices can include a conveyor, for example aroll conveyor with rollers. Other types of systems with or withoutconveyors can also be used. A conveyor can transport substrates into aseries of one or more deposition stations for depositing layers ofmaterial on the exposed surface of the substrate. The deposition chambercan be heated to reach a processing temperature of not less than about450° C. and not more than about 700° C., for example the temperature canrange from 450-550°, 550-650°, 570-600° C., 600-640° C. or any otherrange greater than 450° C. and less than about 700° C. The depositionchamber includes a deposition distributor connected to a depositionvapor supply. The distributor can be connected to multiple vaporsupplies for deposition of various layers or the substrate can be movedthrough multiple and various deposition stations each station with itsown vapor distributor and supply. The distributor can be in the form ofa spray nozzle with varying nozzle geometries to facilitate uniformdistribution of the vapor supply.

Devices including protective layers can be fabricated using soda limefloat glass as a substrate. A film of aluminum-doped ZnO can becommercially deposited by sputtering or by atmospheric pressure chemicalvapor deposition (APCVD). Other doped transparent conducting oxides,such as a tin oxide can also be deposited as a film. Conductivity andtransparency of this layer suit it to serving as the front contact layerfor the photovoltaic device.

A second layer of a transparent conducting oxide, such as tin oxide, ortin oxide with zinc can be deposited. This layer is transparent, butconductivity of this layer is significantly lower than an aluminum-dopedZnO layer or a fluorine doped SnO₂ layer, for example. This second layercan also serve as a buffer layer, since it can be used to preventshunting between the transparent contact and other critical layers ofthe device. The protective layers were deposited in house by sputteringonto aluminum-doped ZnO layers during device fabrication for theseexperiments. The protective layers were deposited at room temperature. Asilicon dioxide capping layer can be deposited over a transparentconducting oxide using electron-beam evaporation.

Devices can be finished with appropriate back contact methods known tocreate devices from CdTe photovoltaic materials. Testing for results ofthese devices was performed at initial efficiency, and after acceleratedstress testing using UV measurements on a solar simulator. Testing forimpact of chemical breakdown in the front contact and protective layerswas done with spectrophotometer reflectance measurements, conductivity(sheet resistance) measurements.

A number of embodiments have been described. Nevertheless, it will beunderstood that various modifications may be made without departing fromthe spirit and scope of the invention. For example, the semiconductorlayers can include a variety of other materials, as can the materialsused for the buffer layer and the protective layer. In another example,additional electrical isolation from cell to cell can be achieved byemploying additional isolation trenches. Accordingly, other embodimentsare within the scope of the following claims.

1. A method for reducing power output rate change variabilitycomprising: receiving photovoltaic power output from a photovoltaicarray; measuring a rate of change of the photovoltaic power output; andadjusting an auxiliary power source output to limit an output power rateof change within a power output rate change band when combined with thephotovoltaic power, wherein the power output rate change band defines amaximum allowable positive and a maximum allowable negative limit forthe plant power output rate of change.
 2. The method of claim 1, whereinadjusting the auxiliary power source output comprises adjusting a poweroutput of a stored energy system.
 3. The method of claim 2, wherein thestored energy system is a high rate rechargeable source with limitedenergy storage.
 4. The method of claim 2, wherein the stored energysystem is a low rate rechargeable auxiliary power source with largeenergy storage.
 5. The method of claim 2, wherein the energy storagesystem uses either a mechanical or electrical device for converting andstoring energy.
 6. The method of claim 2, wherein the stored energysystem is one of an electrolytic cell device, a capacitor, a lead-acidbattery, a nickel-cadmium battery, a sodium battery, a lithium-ionbattery, a flow battery, or a mechanical kinetic and/or potential energystorage device.
 7. The method of claim 2, further comprising increasingan amount of stored energy in the stored energy system when aphotovoltaic output power rate of change exceeds a positive limit of thepower output rate change band.
 8. The method of claim 2, furthercomprising decreasing photovoltaic power when a photovoltaic outputpower rate of change exceeds a positive limit of the power output ratechange band.
 9. The method of claim 2, further comprising maintaining aconstant amount of stored energy in the stored energy system when aphotovoltaic output power rate of change is within the power output ratechange band.
 10. The method of claim 2, further comprising decreasing anamount of stored energy in the stored energy system when a photovoltaicoutput power rate of change exceeds a negative limit of the power outputrate change band.
 11. The method of claim 2, wherein the output powerrate change band is defined by a normal positive power rate change limitand a normal negative power rate change limit.
 12. The method of claim11, further comprising establishing a secondary positive power ratechange limit and a secondary negative power rate change limit.
 13. Themethod of claim 12, further comprising establishing an upper capacitylimit and a lower capacity limit for the stored energy system todetermine the transition from a normal to a secondary power rate changelimit.
 14. The method of claim 12, further comprising establishing anupper capacity hysteresis limit and a lower capacity hysteresis limitfor the stored energy system to determine the transition from asecondary to a normal rate change limit.
 15. The method of claim 12,further comprising switching to the secondary positive power rate changelimit when a level of stored energy falls below a lower capacity limitto facilitate faster charging of the stored energy system when theoutput power rate of change is positive.
 16. The method of claim 12,further comprising returning to the normal positive power rate changelimit when a level of stored energy in the stored energy system risesabove a lower capacity hysteresis limit.
 17. The method of claim 12,further comprising switching to the secondary negative power rate changelimit when a level of stored energy rises above an upper capacity limitto facilitate faster discharging of the stored energy system when theoutput power rate of change is negative.
 18. The method of claim 12,further comprising returning to the normal negative power rate changelimit when a level of stored energy in the stored energy system fallsbelow an upper capacity hysteresis limit.
 19. The method of claim 2,further comprising dispatching plant output power following a dispatchset point.
 20. The method of claim 19, wherein the dispatch set point isconstant.
 21. The method of claim 19, wherein the dispatch set point isderived from a schedule comprising a series of different points.
 22. Themethod of claim 19, further comprising increasing a level of storedenergy in the stored energy system when a photovoltaic output power isgreater than a dispatch set point.
 23. The method of claim 19, furthercomprising lowering a level of stored energy in the stored energy systemwhen a photovoltaic output power is less than a dispatch set point. 24.The method of claim 19, further comprising limiting a rate of change ofa combined photovoltaic power and stored energy system to operate withinthe power output rate change band when a dispatch set point is changed.25. The method of claim 1, wherein adjusting the auxiliary power sourceoutput comprises adjusting a power output of an alternative generationdevice.
 26. The method of claim 25, wherein the alternative generationdevice is one of a fuel cell, a wind turbine generator, a flywheelgenerator, a gas fueled combustion prime mover-generator, a liquidfueled prime mover-generator, a compressed-gas powered primemover-generator, or combinations thereof, including an air-powered primemover-generator.
 27. The method of claim 25, further comprisingmaintaining a constant auxiliary power when a photovoltaic output powerrate of change is within the power output rate change band.
 28. Themethod of claim 25, further comprising reducing auxiliary power when aphotovoltaic output power rate of change exceeds a positive limit of thepower output rate change band.
 29. The method of claim 25, furthercomprising increasing auxiliary power when a photovoltaic output powerrate of change exceeds a negative limit of the power output rate changeband.
 30. The method of claim 25, further comprising decreasingphotovoltaic power when a photovoltaic output power rate of changeexceeds a positive limit of the power output rate change band.
 31. Themethod of claim 25, further comprising maintaining plant output power ata constant level if photovoltaic output power increases or decreases.32. The method of claim 1, further comprising establishing a pluralityof inertial set points to further suppress the rate of plant outputpower rate of change when transitioning from a first power rate changecondition to a second power rate change condition.
 33. The method ofclaim 1, further comprising measuring an irradiance condition proximateto the photovoltaic array and adjusting the plant output power rate ofchange based on the irradiance condition.
 34. The method of claim 33,further comprising reducing the plant output power rate of change whenthe measured irradiance reaches a minimum or maximum value.
 35. Themethod of claim 33, further comprising adjusting the rate at whichstored energy is increased in the stored energy system, up to theallowable positive power rate change limit, when the irradiancemeasurement is increasing.
 36. The method of claim 33, furthercomprising adjusting the rate at which stored energy is decreased in thestored energy system, up to the allowable negative power rate changelimit, when the irradiance measurement is decreasing.
 37. The method ofclaim 33, further comprising adjusting the rate at which auxiliary poweris decreased, up to the allowable positive power rate change limit, whenthe irradiance measurement is increasing.
 38. The method of claim 33,further comprising adjusting the rate at which auxiliary power isincreased, up to the allowable negative power rate change limit, whenthe irradiance measurement is decreasing.
 39. The method of claim 1,further comprising establishing plant operation to follow anenergy/power schedule based on a solar energy production forecast foreach hour of production.
 40. The method of claim 39, further comprisingestablishing an upper and a lower power forecast limit based on theforecasted average power within an hour period.
 41. The method of claim39, further comprising adjusting the auxiliary power or a level ofstored energy in the stored energy system to maintain the plant outputpower rate of change to within the power output rate change band whenplant output power is within the upper and lower power forecast limits.42. The method of claim 39, further comprising decreasing a level ofstored energy in the stored energy system to maintain constant plantoutput power when the plant output power approaches the lower powerforecast limit.
 43. The method of claim 39, further comprisingincreasing the amount of stored energy in the stored energy system tomaintain constant plant output power when the plant output powerapproaches the upper power forecast limit.
 44. The method of claim 39,further comprising increasing the auxiliary power to maintain constantplant output power when the plant output power approaches the lowerpower forecast limit.
 45. The method of claim 39, further comprisingdecreasing the auxiliary power to maintain constant plant output powerwhen the plant output power approaches the upper power forecast limit.46. The method of claim 39, further comprising adjusting auxiliary poweror a level energy stored in the stored energy system to maintain theplant power output rate of change within the power output rate changeband when transitioning from one scheduled hour to the next scheduledhour.
 47. The method of claim 39, further comprising adjusting auxiliarypower or a level of stored energy in the stored energy system to controlthe plant output to minimize any revenue penalties due to the net energydeviation from scheduled based on predetermined criteria associated withutility rate structures and tariffs.
 48. The method of claim 1, furthercomprising adjusting auxiliary power or a level of stored energy levelto compensate for anticipated changes in cloud cover.
 49. The method ofclaim 48, further comprising increasing auxiliary power or a level ofstored energy when cloud cover is forecast or physically detected. 50.The method of claim 48, further comprising adjusting auxiliary power ora level of stored energy in the absence of cloud cover.
 51. The methodof claim 48, further comprising reducing auxiliary power or a level ofstored energy when no cloud cover is forecast or physically detected.52. The method of claim 48, further comprising adjusting auxiliary poweror a level of stored energy for grid frequency regulation, ancillaryservices, or load shifting while producing less variable output from alarge, grid connected photovoltaic plant.
 53. A system of generatingpower of reduced power output rate change variability comprising: aphotovoltaic array; an inverter connected to the photovoltaic array; andan auxiliary power source, wherein the inverter produces alternatingcurrent output power to a grid.
 54. The system of claim 53, furthercomprising a plant control system controlling the inverter and auxiliarypower source, measuring the rate of change of power from thephotovoltaic array, and adjusting the auxiliary power source output tolimit a plant output power rate of change within a power output ratechange band when combined with the photovoltaic power, wherein the poweroutput rate change band defines a maximum allowable positive and amaximum allowable negative limit for the plant power output rate ofchange.
 55. The system of claim 54, further comprising separate andindependently adjustable set points for positive and negative power ratechange limits.
 56. The system of claim 55, further comprising a negativepower rate change limit adjustable between 0 and −100%.
 57. The systemof claim 55, further comprising a positive power rate change limitadjustable between 0 and +100%.
 58. The system of claim 54, furthercomprising set points that can be pre-set and then automatically changedby the plant control system in response to time of day; current,scheduled, or anticipated photovoltaic plant operating conditions; orcurrent or anticipated weather conditions.
 59. The system of claim 54,wherein the auxiliary power source comprises a stored energy system.